This nonprovisional application claims priority under 35 U.S.C. xc2xa7119(a) on Patent Application No. 2001-378946 filed in JAPAN on Dec. 12, 2001, which is herein incorporated by reference.
1. Field of the Invention
The present invention relates to an exhaust emission control device suitable for a diesel engine.
2. Description of the Related Art
An exhaust gas exhausted from a diesel engine contains particulates (soot) mainly composed of carbon, and various techniques for suppressing this soot to be exhausted to the atmosphere have been developed.
As one of those techniques, an exhaust emission control device of the continuous regeneration type is well-known in which an oxidation catalyst and a diesel particulate filter (DPF) are disposed in succession from the upstream side in an exhaust pipe of the diesel engine.
In this exhaust emission control device, NO in the exhaust gas is oxidized by the oxidation catalyst to produce NO2 (2NO+O2xe2x86x922NO2), and the soot (carbon: C) captured in the DPF is oxidized by NO2 produced by the oxidation catalyst (NO2+Cxe2x86x92NO+CO, 2NO2+Cxe2x86x922NO+CO2). In this exhaust emission control device, NO contained greatly in the exhaust gas is oxidized and converted into NO2, which has a high function as an oxidant, and the soot is combusted at relatively low exhaust gas temperatures owing to this oxidation function of NO2.
Thereby, it is possible to remove the deposited soot, and regenerate the DPF continuously, utilizing the temperature of the exhaust gas itself, without the use of other heating apparatuses such as an electric heater or a burner.
The conventional exhaust emission control device can combust and remove the soot at relatively low exhaust gas temperatures owing to catalytic action. Thus, its exhaust emission control function depends on the oxidizing ability of the oxidation catalyst. Since this oxidizing ability typically depends on the temperature of the exhaust gas surrounding the oxidation catalyst, the soot is not fully removed and the residual soot is accumulated on the DPF when the exhaust gas temperature is below a predetermined temperature that is required for the soot combustion. This occurs in a low load area of the engine.
If the soot is excessively deposited on the DPF, there is a risk that the engine output is decreased and also the DPF melts away due to abnormally high temperatures during the soot combustion. Therefore, it is required to remove the deposited soot by any means, and such methods for removing the soot include a forced regeneration method and a continuous regeneration assist method, which we reconventionally developed and adopted.
The forced regeneration method involves elevating the exhaust gas temperature up to a temperature range required for O2 combustion of the soot due to post injection or the like and forcedly combusting the soot by O2. However, this method is inferior in the fuel consumption because excess fuel is consumed, and must be improved for the practical use.
The continuous regeneration assist method involves assisting a continuous regeneration function by elevating the exhaust gas temperature to the maximum conversion temperature range at which the NO2 conversion efficiency of oxidation catalyst is at peak by an exhaust throttle, and exhibiting the catalytic ability of oxidation catalyst to the maximum. However, with this method, since a variation in the exhaust gas temperature is gentle, it takes a long time to elevate the temperature, greatly aggravating the fuel consumption, when the exhaust gas temperature greatly falls below the maximum conversion temperature range.
An object of the present invention is to provide an exhaust emission control device that can regenerate a filter without greatly aggravating the fuel consumption.
An exhaust emission control device of the present invention has a filter disposed in an exhaust passage of an engine for capturing particulates in an exhaust gas, and a catalyst disposed in the exhaust passage on the upstream side of the filter for converting NO in the exhaust gas into NO2, in which the particulates deposited on the filter are combusted by NO2 converted by the catalyst, thereby controlling the exhaust emission of the engine.
In order to achieve the above object, the exhaust emission control device comprises deposited amount detecting means for detecting a deposited amount of particulates deposited on the filter, operating state detecting means for detecting an operating state of the engine, temperature sensing means for sensing the temperature of the catalyst or the exhaust gas temperature on the upstream side of the filter, regeneration assist means for controlling the exhaust gas temperature or the temperature of the catalyst to enhance the conversion efficiency of the catalyst into NO2, forced regeneration means for controlling the exhaust gas temperature or the temperature of the filter to be higher than the temperature that enhances the conversion efficiency of the catalyst into NO2, and control means for activating the regeneration assist means, if it is determined that a specific operating state involving incomplete combustion of particulates by the NO2 is continued for a predetermined period or more as a first determination condition on the basis of the detected information by the operating state detecting means, or activating the forced regeneration means, if it is determined that the deposited amount of particulates detected by the deposited amount detecting means exceeds a predetermined value as a second determination condition, wherein the control means activates either the forced regeneration means or the regeneration assist means in accordance with the temperature sensed by the temperature sensing means, when it is determined that the first and second determination conditions are established at the same time.
In this exhaust emission control device, the control means activates either the forced regeneration means or the regeneration assist means in accordance with the temperature sensed by the temperature sensing means, when it is determined that the first determination condition for activating the regeneration assist means and the second determination condition for activating the forced regeneration means are established at the same time.
When the first and second determination conditions are established at the same time, either the regeneration assist means or the forced regeneration means is activated, so that an optimal control can be effected in accordance with an engine operating situation. Namely, an effective filter regenerating process can be made.
It is desirable that the control means activates the regeneration assist means if the temperature sensed by the temperature sensing means is higher than or equal to a predetermined temperature that is lower than the temperature to enhance the conversion efficiency of the catalyst into NO2, or activates the forced regeneration means if the temperature sensed by the temperature sensing means is below the predetermined temperature, when it is determined that the first and second determination conditions are established at the same time.
Herein, when the first and second determination conditions are established at the same time, and if the temperature sensed by the temperature sensing means is higher than or equal to the predetermined temperature that is lower than the temperature to enhance the conversion efficiency of the catalyst into NO2, the control means activates the regeneration assist means, because there is a small difference between the exhaust gas temperature or the temperature of the catalyst and the temperature to enhance the conversion efficiency of the catalyst into NO2. Thereby, it takes a shorter time to control (elevate) the temperature, and it is possible to continuously regenerate the particulates owing to catalytic action of the catalyst, with reduced consumption energy.
Moreover, when the control means activates the forced regeneration means if the temperature sensed by the temperature sensing means is higher than or equal to the predetermined temperature that is lower than the temperature to enhance the conversion efficiency of the catalyst into NO2, the exhaust gas temperature is so high that the filter may melt away. However, with the invention, it is possible to prevent the filter from melting away.
Also, the control means activates the regeneration assist means if the temperature sensed by the temperature sensing means is below the predetermined temperature that is lower than the temperature to enhance the conversion efficiency of the catalyst into NO2, there is a large difference between the exhaust gas temperature or the temperature of the catalyst and the temperature to enhance the conversion efficiency of the catalyst into NO2, in which it takes a longer time to control (elevate) the temperature, with increased consumption energy. Hence, the exhaust gas temperature or the temperature of the filter is controlled (elevated) to the temperature for O2 combusting the filter higher than the temperature to enhance the conversion efficiency of the catalyst into NO2, to combust the particulates, whereby the consumption energy can be reduced as compared with the regeneration assist means.
It is desirable that the control means inhibits the activation of the regeneration assist means, if the first determination conditions is established and it is determined that the engine is in an accelerating state on the basis of the detected information by the operating state detecting means.
In this accelerating state, an excess air ratio xcex is controlled to be smaller, or in other words, the air fuel ratio is controlled to the rich side, so that the exhaust gas temperature rises to maximize or enhance a catalytic action of the catalyst. Thereby, it is expected that the combustion is promoted owing to the continuous regeneration of particulates. Therefore, it is possible to prevent the wasteful energy consumption by inhibiting the activation of the regeneration assist means in the accelerating state.
It is desirable that the control means determines that the second determination condition is established, if the deposited amount of particulates detected by the deposited amount detecting means exceeds a predetermined value, and the engine is in a specific operating state.
Namely, if the deposited amount of particulates detected by the deposited amount detecting means exceeds the predetermined value, and the engine is in the specific operating state involving incomplete combustion of particulates by NO2, it is determined that the second determination condition is established, and the forced regeneration means is activated.
When the engine is in the specific operating state involving incomplete combustion of particulates by NO2, the forced regeneration means is activated to control (elevate) the exhaust gas temperature or the temperature of the filter to be higher than the temperature to enhance the conversion efficiency of the catalyst into NO2, whereby it is possible to avoid the forced regeneration control in the operating state where the particulates deposited on the filter can be sufficiently combusted by NO2, or the continuously regenerable area in which the particulates are continuously combustible. Therefore, the activation interval is lengthened, the activation frequency is reduced, and the wasteful energy consumption is prevented.
It is desirable that the temperature sensing means is disposed on the upstream side of the filter and on the downstream side of the catalyst and senses the temperature of the exhaust gas that passed through the catalyst.
Since the temperature sensing means is disposed on the upstream side of the filter and on the downstream side of the catalyst and senses the temperature of the exhaust gas after passing through the catalyst, a difference between the exhaust gas temperature or the temperature of the catalyst and the temperature to enhance the conversion efficiency of the catalyst into NO2 can be measured precisely. Therefore, it is possible to reduce the consumption energy in filter regeneration and prevent the filter from melting away.
It is desirable that the control means inhibits the activation of the regeneration assist means, if the first determination conditions are established and it is determined that the engine is in a decelerating state on the basis of the detected information by the operating state detecting means and the suction throttle or exhaust throttle is on the closed valve side.
Namely, even in the state where the first determination condition is established, the suction air amount is restricted because the suction throttle or exhaust throttle is controlled on the closed valve side in the decelerating state, whereby the exhaust gas temperature is prevented from falling, the conversion of the catalyst into NO2 is sufficiently made, and the combustion of particulates deposited on the filter by NO2 is promoted. Therefore, it is possible to prevent the wasteful energy consumption by inhibiting the control (temperature elevation activation) for the exhaust gas temperature or the temperature of the catalyst by the regeneration assist means in the decelerating state.